Composite cardboard containers or cans generally comprise a multilayer composite envelope, sleeve or shell, a base placed on one end of the shell and a cover or lid. In the case of a liquid-tight and aroma-tight construction, a sealing membrane is sealed onto the lid side.
The composition of the composite material is a function of the filling material and the given strength requirements. In the case of a liquid-tight construction, the shell has at least one paper layer, on whose inside is provided a metallic barrier layer, generally an aluminium foil, which is in turn lined with a polymer layer. On said shell is externally provided at least one further, generally thicker cardboard layer, the so-called screening, and on the latter is finally placed a label-forming paper web. These shells are constructed as cores, which are cut to length from a helically, continuously produced laminated tube.
In the manufacture of a core, the composite material passes as an endless web onto a winding mandrel under an angle corresponding to the pitch of the helical package. Along one longitudinal edge of the continuous web, a marginal strip is bonded to the marginal strip below it of the section already located on the winding mandrel. Such a core does not satisfy the increased demands concerning gas tightness. The durability of the composite can also suffer in a humid atmosphere. The reason for this is that the externally and internally positioned longitudinal edge form an open surface of cut, so that diffusion processes can take place through the paper or cardboard layer via the surfaces of cut. The can is neither gas-tight, nor pressure-tight. Moisture diffusing in can lead to the swelling of the paper or cardboard and to delamination.
These diffusion processes can be eliminated by a so-called anaconda seam. In this process the continuous web is wrapped round outwards to a double-layer marginal strip along that longitudinal edge, which is subsequently placed on the inside of the finished core (U.S. Pat. No. 3,716,435, EP 113 160). The opposite, undeformed longitudinal edge is heated, so that the polymer layer melts in a marginal strip. In the case of helical winding by means of a winding device, said heated marginal strip passes onto the outer layer of the double layer on the other longitudinal edge and its inside, heated polymer layer welded to the outer polymer layer on the double layer. On said core is then wound at least one further cardboard layer. This prevents diffusion processes from the inside to the outside or vice versa, because on the inside the shell has no free surface of cut and instead a through, closed polymer layer. The seal or tightness of such a core is then substantially only dependent on the welding seam quality.
Due to the incorporated aluminium film, the in trinsically favourable, because very readily controllable high frequency welding cannot be used for welding the polymer layers. Thus, hitherto exclusively hot air has been used and is transported by means of nozzles to the seam (U.S. Pat. No. 3,716,435, EP 113 160). However, this only permits a very poor, local temperature concentration. On impact with the continuous web, the hot air is also deflected to the side, so that a relatively wide strip is heated. In addition, in the case of helical or spiral winding, the hot air must be blown from below onto the continuous web immediately prior to its passing onto the winding mandrel, so that the migration of heat into neighbouring areas is increased. Consequently the polymer layer melts not only at the desired seam, but also in the marginal area. Finally, the temperature at the seam can only be poorly controlled, so that in certain circumstances the polymer melt melts excessively and migrates and optionally also passes onto the winding mandrel, where it sticks. Similar conditions occur with heating by infrared radiation and once again only an inadequate temperature control is possible. The continuous web also takes up too much heat in an excessively large area and then the welding seam cools too rapidly. As a result of the inadequate heat transfer and its poor control possibility, the web speed is limited and consequently so is the output, i.e. the number of shells which can be manufactured per unit of time. Due to the high heat dispersion and thermal losses, the energy needs are also considerable.
It is also known to join thermoplastics or thermoplastically coated composite materials by focussed thermal radiation, so that a better energy balance is obtained. Thus, it is known in connection with cardboard boxes (U.S. Pat. No. 4,156,626) to subject one of the bottom or lid tabs, which are internally coated with a polymer and placed virtually perpendicularly during transportation, to a light source, whose radiation is focussed in punctiform manner. The radiation focus is oriented. on the polymer layer. A strip of the polymer layer is melted on when conveying past the boxes. Subsequently the bottom or lid tab is placed round the other tab and connected thereto by the molten strip.